Process for making sheet from brittle metals



March 23, 1965 D. H. BLAIR 3,174,221

PROCESS FOR MAKING SHEET FROM BRITTLE METALS Filed Dec. 20, 1960 D 1 S H. Blair QUE a INVENTOR.

United States Patent 3,174,221 PROCESS FOR G SHEET FRQM BRITTLE METALS Douglas H. Blair, Monmouth, Greg, assignor to Oregon Metallurgical Corporation, Albany, Greg, 2 corporation of Oregon Filed Dec. 20, 1966, Ser. No. 77,153 5 Claims. (Cl. 29--528) This invention relates to the production of metal sheet, and more particularly to a novel method for producing sheet from refractory metals characterized by brittleness at forging temperatures (the term meta including both pure and alloyed metals). The method has particular utility in connection with metals improved by vacuum melting.

Materials such as molybdenum, columbium, and tungsten, and refractory alloys of these and other materials such as Waspaloy (containing about 19.5% chromium, 57% nickel, 13.5% cobalt, 4.3% molybdenum, 3.1% titanium, 1.3% aluminum, 1.2% iron, and traces of carbon), Udimet 500 (containing about 19% chromium, 19% cobalt, 4.0% molybdenum, 3.0% titanium, 3.0% aluminum, 0.1% carbon, and the balance nickel), and Chrornel (a nickel-chromium alloy) have very little ductility when at forging temperatures. Typically, when a test specimen selected from such a group of materials is subjected to the usual tensile strength tests at temperatures where it is to be forged or milled, it breaks apart sharply as tension is applied thereto, with substantially no reduction of cross section and substantially no elongation occurring in the test specimen. This invention is applicable to metals where reduction in cross section and elongation in a test specimen during such tensile strength tests are less than about and is particularly applicable to metals where such reduction and elongation are in the range of less than about 2%.

A general object of the invention is to provide an improved method for manufacturing sheet from such materials, where substantially higher yields of sheet are possible for a given mass of starting material prior to casting than previously possible. Using the process of the invention, relatively small amounts of metal are trimmed during the making of sheet, and it is not uncommon to obtain a sheet product that represents some 90% by weight of the original casting.

Another general object is to provide an improved method for manufacturing sheet, wherein the number of handling steps employed in making the sheet are considerably reduced from those required using prior known techniques. This feature of the invention results in savings in labor and capital equipment costs.

Still another object is to provide an improved method for manufacturing sheet from refractory-type metals wherein an ingot or billet is formed early in the process having improved grain structure and forgeability characteristics, making unnecessary subsequent extrusion or similar processes in order to place the metal in condition for final sheet fabrication.

A further object is to provide a process for the manufacture of sheet wherein practical difficulties in handling the material that heretofore have been major problems are substantially eliminated. Thus, manufacturing techniques that have the effect of producing a better product and that contribute to ease of handling are incorporated into the process without the drawbacks normally associated with the techniques.

These and other objects and advantages are attained by the invention, and the same is described hereinbelow in conjunction with the accompanying drawings, wherein:

FIG. 1 is a simplified drawing illustrating diagrammatically a furnace wherein metals may be reduced to a ice molten state and then oast within the furnace, in the manner contemplated by an embodiment of this invention;

FIG. 2 illustrates an ingot or billet formed as an intermediate product in the process contemplated, such being used in the production of metal sheet;

FIG. 3 illustrates one half of the billet after cutting into two pieces, to prepare the same for drawing between the rolls of a rolling mill;

FIG. 4 illustrates the drawing of such a piece in a rolling mill; and

FIG. 5 illustrates the drawing out of an annular ingot without severing the same.

In general terms, this invention relies on techniques which produce, as a product for further processing in the manufacture of sheet, (A) an ingot where the metal of the ingot is cooled rapidly with the production of equiaxed grain in the ingot as opposed to the columnar grain that results when a large mass of metal is cooled slowly; (B) an ingot where the metal of the ingot has approximately true density i.e., an ingot free of porosity; (C) an ingot which has a limited thickness (as a practical matter, the maximum thickness of any portion of the ingot should not exceed about three inches, since a greater thickness results in tensions in the ingot when it is flattened out producing fissures in the ingot, and also affects the rate of cooling and the type of grain in the ingot); (D) an ingot shaped so that a relatively large mass of material can be processed at once while keeping wall thickness within the range desired and without excessive bulk in the ingot (in this connection, the ingot is made in the shape of a hollow cylinder, the cylindrical wall of the ingot containing as much metal as a considerably larger ingot formed as a flat piece); and (E) an ingot shaped so that it may be drawn through the rolls of a rolling mill relatively easily and without intermediate processing (thus the cylindrical, relatively thin-walled ingot is prepared so that its inner radius is larger than the radius of the rolls of the rolling mill, which enables the cylindrical wall of the ingot to be drawn through the rolling mill rolls directly and without tensions being produced and fissures occurring). As a further feature of the invention, the ingot is prepared in such a manner that pouring of the same is practical, and detrimental premature solidification of molten metal during pouring is inhibited.

Explaining now specifically the process contemplated, initially a melt is prepared. Conventional types of furnaces may be used in doing this. Thus, high melting point metals may be reduced to molten form using either an electric arc furnace, or an induction furnace. With metals that are highly reactive at melting temperautres (i.e., with metals such as molybdenum, tungsten, alloys of these metals, etc.), the melting is carried out in an inert environment. Thus, either a vacuum furnace may be used, or the melting may be done in an atmosphere of inert gases (argon, helium, etc.). For the purpose of illustration, the invention is described below in connection with the production of metal sheet from molybdenum using an electric arc, vacuum furnace, although it should be understood that the particular type of furnace used and the environment in which the melting takes place is variable depending upon the facilities available and the reactivity of the type of metal handled.

With reference to FIG. 1, 20 indicates portions of a furnace chamber in a vacuum furnace, such being completely enclosed and having an interior sealed from the atmosphere in any suitable manner. Within furnace 20 is a consumable electrode 24 of molybdenum. During operation of the furnace, electrode 24 is melted by means of an electric arc, and metal from the electrode is collected in water-cooled copper crucible 26 shown in an upright position in solid lines in FIG. 1. The copper crucible is also an electrode in the furnace and an arc Patented Mar. 23, 1965 is struck between electrode 24 and the crucible by passing a realtively high amperage, low voltage current between the two electrodes. The means for producing a vacuum within the furnace chamber, details of crucible 26 and its water cooling, and details of the electrical connection for the two electrodes are not illustrated in FIG. 1, as such are known in the art. Reference is made to the disclosure of a casting furnace made in Aschoff, US. 2,912,476.

After melting of the electrode and collection of molten metal, crucible 26 is tilted (as shown by the dashed lines for the crucible), and the molten metal therein poured through a funnel 27 into a centrifugal mold 30 Where the metal is cast into an ingot or billet. Mold 30 has an upright, cylindrical mold cavity 32 bounded by a vertical cylindrical wall 33 that is concentric with the rotation or spin axis for the mold, indicated at 34. A centrifugal mold, as used herein, refers to a mold where rotation of the mold takes place about an axis extending through the center of the mold cavity, as compared to a centrifuge mold where the rotation axis is off to one side of the mold cavity.

The shape of the mold is important, if maximum yields are to be obtained by the process of the invention. With the upright cylindrical mold cavity shown, as will hereinafter become apparent, a final hollow cylindrical ingot or billet is formed, where the outer surface of the billet is concentric with the center axis of the mold cavity and the axis of the interior of the billet. When such a billet has its outer surface machined to remove contaminants and to smooth the surface, a cylindrical product may be produced with a finished outer surface concentric with the hollow interior thereof, with a minimum of metal removed from the outside of the billet.

At the top of mold 30 is an opening 35 which enables metal to be poured into cavity 32. Opening 35 is defined by a shoulder 36. A mold with a vertically disposed rotation axis 34 is used, since it has been found that such an organization considerably simplifies problems of pouring molten metal into the mold. Transfer of metal to the mold is-easily achieved without premature solidifying of the metal, or excessive retention of metal in any transfer means between the crucible and the entrance to the mold cavity. Were the mold to have a horizontal rotation axis, for instance, material would have to be introduced to the mold through one end, and it should be obvious that this would result in serious diflicult-ies. The vertical position of the mold rotation axis is also important, in that it makes practical the construction of a mold with a mold cavity of relatively large diameter. Molds with mold cavities some two feet or so in diameter have been used in the practice of this invention, and these may be mounted relatively easily in a vertical position with 'the mold stable at all running speeds.

The molten metal is poured into mold 30 preferably with the mold spinning. The mold is continuously spun until metal within the mold has completely solidified. While the pour could be completed before spinning of the mold was started, such would tend to produce a mass of solidified metal at the base of the mold cavity, which is undesirable. According to this invention, the mold is rotated or spun at a speed suflicient to cause metal after it enters the mold cavity to climb the sides of the mold cavity and become substantially evenly distributed between top and bottom ends of the mold cavity. In this Way the thickness of the cylindrical wall of the billet formed is substantially the same at axially top and bottom ends. By pouring the metal into the mold cavity as it spins, metal moves immediately to the sides of the mold cavity with a minimum amount of solidification at the bottom of the cavity. In the drawings, a means for rotating mold 30 is represented by motor 36'.

It has been discovered that satisfactory distribution of metal results if the mold is rotated at a sufficient speed to produce preferably a centrifugal force acting on the metal within the mold that ranges upwardly from about ten times the force of gravity. For optimum results, a speed producing a centrifugal force ranging upwardly from about thirty times the force of gravity is desirable. With such speeds, in a mold about a foot high, the difference between the thickness of the wall of the billet that forms at the top and bottom of the mold may be kept within about one-eighth of an inch. Optimum densification of the metal also results. With slower speeds, the cylindrical wall of the hollow billet that forms tends to be appreciably thicker at the bottom than at the top (the inner wall slopes along a parabolic curve). Rotation speeds that may be used, of course, are to some extent dependent upon the conductivity of the metal handled, its heat of fusion, its density, and its viscosity.

Mold 30 in which the metal is collected may be comprised of graphite, since such is an economical material to be used for this purpose. The temperature of the mold at the time metal first is poured into the mold ordinarily is approximately room temperature (which is substantially below the heat of fusion of the metal). The metal on contacting the graphite walls of the mold and distributing itself about the interior of the mold, rapidly cools, first forming an outer shell, the wall of which progressively thickens as heat is dissipated from the metal into the walls of the mold..

The amount of metal poured into the mold .is regulated, so that the cylindrical wall of the hollow billet that forms is relatively thin. The maximum thickness of the ingot wall should not exceed about three inches. Preferably, the wall thickness should not exceed about two inches. The two inch maximum thickness is preferable, since this enables rapid cooling of the metal in the billet, whereby the grain of the metal in the solidified billet is equiaxed, and not columnar. A greater thickness also produces difiiculties in forging, since flattening of-too thick a piece produces tensions that result in fissures. 'I'he graphite mold preferably has relatively thick walls (commonly the walls are some three inches or so in thickness), and thus the mold functions as a massive heat sink. Solidificationof the metal ordinarily is complete in less than about two minutes.

With materials such as molybdenum and tungsten, the steps of melting, pouring and solidification of the metal are carried out in an inert environment. This inhibits contamination, as the materials are quite reactive at raised temperatures. After an ingot cools, it may be removed from the furnace and exposed to the atmosphere without harmful contamination occurring. With high melting point metals that tend to solidify rapidly, especially during the pouring into a spinning mold, a vacuum may be selected as the inert invironment used in the melting, pouring and solidification of the metal. With a vacuum, premature loss of heat into the medium forming the environment of the furnace is minimized.

According to this invention, after the ingot has solidified and has been separated from the mold, its outer surface is machined as by turning it on a lathe, to remove any contaminants on the outer surface. resulting from contact with the graphite mold, and to smooth the outer surface. While the inner substantially cylindrical surface of the hollow ingot is substantially free of contaminants, preferably it also is machined so as to smooth it and make the cylindrical wall of the ingot truly uniform in thickness throughout. The result is a finished cylindrical ingot with clean, smooth inner and outer surfaces, such as that illustrated in FIG. 2 at 40.

After the ingot or billet has been machined, preferably it is cut parallel to its axis to produce pieces of finite length extending in the direction of curvature of the billet. Such a piece is shown in FIG. 3 and indicated as 42. Piece 42 canbe drawn or flattened directly into sheet, using a rolling mill. First, of course, the metal is heated to forging temperature, if such is necessary. While columbium can be rolled at room'tem'perature, tungsten and molybdenum ordinarily are rolled when heated to temperatures of about 3000 F. and 2000 F., respectively.

Rolling of a piece 42 is illustrated in FIG. 4. The piece is passed in a direction extending transversely of its axis of curvature between rolls 46, 48 of the rolling mill. As can be seen in FIG. 4, the axis of curvature of the piece parallels the axes of the rolls in the rolilng mill. It will be noted that the radius of curvature of piece 42 exceeds the radius of curvature of rolls 46, 48. With roll 46 that is on the concave side of the piece at is enters between the rolls of smaller radius than the radius of curvature of the piece, binding of the trailing end of the piece is prevented. The material leaves the bite of the rolls lying substantially flat. By continued rolling, a sheet of desired thickness may be formed. The final sheet is trimmed along ends and edges in a conventional manner.

When rolling a piece such as piece 42, the amount of flattening which results on each pass through the rolls depends upon the amount of work performed by the rolls. Cylindrical ingot 40 may be rolled and drawn out, without severing the ingot to make two pieces, as shown in FIG. 5. The cylindrical wall of the ingot is passed in a direction extending transversely of its axis of curvature between rolls as shown in FIG. 5, and again the radius of curvature of the ingot should exceed the radius of curvature of the rolls. The material on leaving the rolls will not be flat, as indeed it cannot be since the cylindrical wall of the ingot is continuous. Flattening will occur during the rolling operation, as the curvature in any given portion of the wall of the ingot each time it passes between the rolls will decrease, the amount such curvature decreases depending upon the amount of Work performed in drawing out the ingot, more flattening occurring in those instances where the most work is performed and greatest reduction of wall thickness is produced.

Having described the invention, it should be apparent that there are several features and advantages that distinguish it. By using a vertical centrifugal mold, a hollow ingot is formed that has a substantially noncoutarninated inner surface. Very important, the ingot cools rapidly and the metal of the ingot has equiaxed grain, as opposed to columnar grain. Using old techniques, it is frequently necessary with brittle metals to perform an extrusion step before the metal can be milled, if fracturing is to be prevented. Such extrusion is unnecessary with the ingot of the process contemplated.

Casting of the annular billet or ingot is made easy thruough the vertical arrangement of the mold spin axis, as it enables the metal readily to be poured directly into the mold while it is spinning. An even wall thickness is produced through the expedient of rotating the mold at a sufiicient speed to cause the metal to climb the walls of the mold and distribute itself properly.

While considerable mass of metal is contained in the ingot, its cylindrical wall is relatively thin. There is no requirement that a cogging or upset forging operation be carried out before rolling.

Modifications and variations of the invention are possible, and it is desired not to be limited specifically to the embodiment of the invention disclosed. Intended to be covered are all modifications. and variations that would be apparent to one skilled in the art, that come within the scope of the appended claims.

I claim:

1. A process for the manufacture of sheet from refractory metals characterized by brittleness at forging temperatures comprising first forming an annular billet by the steps of melting the metal, pouring the molten metal so formed into a vertically disposed mold cavity of a substantially cylindrical centrifugal mold having a vertical spin axis and walls substantially below the fusion temperature of the metal, cooling the metal so that it solidifies Within the mold, while pouring and cooling the metal spinning the mold at a speed sufficient to distribute the metal substantially evenly between the top and bottom ends of the mold said melting, pouring and cooling steps being done in an inert environment; removing the billet from the mold; cleaning and smoothing the inner and outer circumferential surfaces of the annular billet formed by machining the surfaces; after cleaning and smoothing said surfaces severing the billet in a direction paralleling its axis to form at least one piece of finite length extending in the direction of curvature of the billet; and stretching out such piece by passing it through the opposed rolls of a rolling mill, said stretching being done by passing the piece between the rolling mill rolls with the piece traveling in a direction extending transversely of its axis of curvature and with the pieces axis of curvature parallel to the axes of the rolling mill rolls, said stretching of the piece being performed with a roll on the concave side of the piece as it enters between the rolls having a radius less than the radius of curvature of the piece and without any straightening of the curvature in the piece before the same enters between the rolling mill rolls, whereby tensioning of the concave side of the piece and fissures on this side are prevented.

2. The process of claim 1 wherein the mold is spun at a speed sufiicient to produce a centrifugal force on the metal within the mold that is more than thirty times the force of gravity.

3. The process of claim 1 wherein the molten metal poured into the mold is regulated so that the annular billet formed has a maximum wall thickness of three inches.

4. In the manufacture of sheet from metals characterized by brittleness at forging temperatures, the process comprising melting the metal in an inert environment, collecting the molten metal while in an inert environment in a centrifugal mold, cooling the metal whereby the same solidifies in an inert environment and while spinning the mold to produce a centrifugal force that is more than thirty times the force of gravity, the molten metal poured into said mold being controlled whereby the mold is only partially filled and an annular billet is formed on spinning of the mold having a wall thickness not exceeding three inches, removing the annular billet formed from the mold, machining the inner and outer circumferential surfaces of the annular billet to remove contaminants and porosity on the surfaces thereof, after said machining severing the billet in a direction paralleling its axis to form at least one curved piece of finite length extending in a direction of the curvature of the billet, and stretching out such curved piece by passing it through the opposed rolls of a rolling mill, said stretching being done by passing the piece between the rolling mill rolls with the piece traveling in a direction extending transversely of its axis of curvature and with the pieces axis of curvature parallel to the axes of the rolling mill rolls, said stretching of the piece being performed with a roll on the concave side of the piece having a radius less than the radius of curvature of the piece and without any straightening of the curvature in the piece before the same enters between the rolling mill rolls, whereby tensioning of the concave side of the piece and tissues on this side are prevented.

5. A process for the manufacture of sheet from refractory metals characterized by brittleness at forging temperatures comprising first forming an annular billet with a substantially cylindrical wall by the steps of melting the metal, pouring the molten metal so formed into a vertically disposed mold cavity of a substantially cylindrical centrifugal mold having a vertical spin axis and walls substantially below the fusion temperature of the metal, cooling the meta-1 so that it solidifies within the mold, and while pouring and cooling the metal spinning the mold at a speed suflicient to distribute the metal substantially evenly between the top and bottom ends of the mold; said melting, pouring, and cool-ing steps being done in an inert environment; removing the billet from the mold; cleaning and smoothing the inner and outer circumferential surfaces of the cylindrical wall of the annular billet formed by machining the surfaces; and stretching out the cylindri- 7 cal wall of the annular billet by pasisng such wall through the opposed rolls of a rolling mill; said stretching being done by passing the wall between the rolling mill rolls with the wall traveling in a direction extending transversely of its axis of curvature and with the walls axis of curvature parallel to the axes of the rolling mill rolls; said stretching of the wall being performed with a roll on the concave side of the wall as it enters between the rolls having a radius less than the radius of the wall of the billet and without straightening of the curvature in the wall before the same enters between the rolling mill rolls, whereby tensioning of the concave side of the wall and fissures on this side are prevented.

References Cited by the Examiner UNITED STATES PATENTS McMullan 2918 2,055,171 9/36 Dailey 29-33 2,087,765 7/37 Schermer 80-60.5X 2,087,766 7/37 Forbes et al. 80--60.5X 2,087,768 7/37 Forbes et al. 80-60.5X 2,266,972 12/41 Holmquist 80--60.7X 2,327,209 8/43 Otte 29-18X 2,681,485 6/54 Smith 222005 2,698,267 12/54 Halversen 2,835,022 5/58 Harris 80-60.7X 2,912,476 11/59 Asc'hoff 13-10 3,052,001 9/62 Brennan 22200.5X 3,116,524 1/64 Royal 22--2l6.5

FOREIGN PATENTS 247,507 3/26 Great Britain.

OTHER REFERENCES Hollow Moly Cylinder, The Iron Age, page 24, J an. 22, 1959.

20 WHITMORE A. WILTZ, Primary Examiner.

THOMAS E. BEALL, Examiner. 

5. A PROCESS FOR THE MANUFACTURE OF SHEET FROM REFRACTORY METALS CHARACTERIZED BY BRITTLENESS AT FORGING TEMPERATURES COMPRISING FIRST FORMING AN ANNULAR BILLET WITH A SUBSTANTIALLY CYLINDRICAL WALL BY THE STEPS OF MELTING THE METAL, POURING THE MOLTEN METAL SO FORMED INTO A VERTICALLY DISPOSED MOLD CAVITY OF A SUBSTANTIALLY CYLINDRICAL CENTRIFUGAL MOLD HAVING A VERTICAL SPIN AXIS WALL SUBSTANTIALLY BELOW THE FUSION TEMPERATURE OF THE METAL, COOLING THE METAL SO THAT IT SOLIDIFIES WITHIN THE MOLD, AND WHILE POURING AND COOLING THE METALSPINNING THE MOLD AT A SPEED SUFFICIENT TO DISTRIBUTE THE METAL SUBSTANTIALLY EVENLY BETWEEN THE TOP AND BOTTOM ENDS OF THE MOLD; SAID MELTING, POURING, AND COOLING STEPS BEING DONE IN AN INERT ENVIRONMENT; REMOVING THE BILLET FROM THE MOLD; CLEANING AND SMOOTHING THE INNER AND OUTER CIRCUMFERENTIAL SURFACES OF CYLINDRICAL WALL OF THE ANNULAR BILLET FORMED BY 